1. Field of the Invention
The invention relates to a method for operating a MIMO radar, e.g., a use of a MIMO radar in the automotive sector.
2. Description of the Related Art
Radar systems are used in motor vehicles in order to determine distances and relative velocities of objects in the vicinity of the motor vehicles. The aforesaid radar systems can be used in order to implement convenience functions (for example adaptive cruise control [ACC]) and safety functions (for example to warn the driver in critical situations or to cause maximum braking when a collision can no longer be avoided).
For these applications, the distance and the relative velocity of the objects, and the angle of the reflected radar waves, must be precisely identified. An exact estimate of the angle, or direction of arrival (DOA), should in particular be carried out for this purpose. The angle to be estimated can be an azimuth angle and/or an elevation angle. Multiple-input/multiple-output (MIMO) radar systems having multiple transmitting and receiving antennas offer the advantage, as compared with conventional single-input/multiple-output (SIMO) radar systems, of a larger virtual aperture for the same or smaller geometric size. Greater accuracy for the estimated DOA can therefore normally be achieved with a MIMO radar.
In general, however, the accuracy of DOA estimates in a MIMO radar decreases when the object is moving relative to the radar, since the motion generates an unknown phase change (Doppler phase) in the baseband signal because of the Doppler effect.
R. Feger, C. Wagner, S. Schuster, S. Scheiblhofer, H. Jäger, A. Stelzer: “A 77-GHz FMCW MIMO Radar Based on a SiGe Single-Chip Transceiver,” 2009, discloses a MIMO radar in which receiving and transmitting antennas are positioned in such a way that at least two antenna elements of a virtual array have the same geometric position (“redundant position”). An unknown phase change (Doppler phase) in the baseband signal due to a target motion is estimated by calculating the phase difference of the antenna elements at the redundant positions. The estimated Doppler phase is then used as a correction in an algorithm for angle estimation.
Only antenna elements at the redundant positions are used, however, to identify the unknown Doppler phase. Then all the antenna elements are used in order to identify the angle at which the signal is arriving. Noise in the antenna elements at the redundant positions therefore has a greater influence on the DOA estimate than noise in the remaining antenna elements. Depending on the time multiplex schema used, and on the disposition of the antennas, a DOA estimate of such a system can in fact be poorer than utilization of an individual transmitting antenna.
M. Wintermantel: “Radar System with Elevation Measuring Capability,” 2010, discloses a MIMO radar system having multiple transmitting and receiving antennas. The transmitting antennas transmit numerous short-duration chirp sequences, for example 512 or 1,024 chirps each having a duration of, for example, 10 μs. The transmitters transmit sequentially or simultaneously, but with different phase modulations. The spacing, relative velocity, and DOA are estimated using a three-dimensional discrete Fourier transform. The proposed system can carry out only inaccurate angle estimates, however, and furthermore cannot be used at all with a frequency modulated continuous wave (FMCW) radar having long ramps. The hardware is moreover relatively complex and laborious to implement because of the chirp-sequence principle with rapid frequency changes.